Memcapacitive charge memory in inverse spinel NiFe₂O₄ enabled by defect-assisted dielectric polarization

Memcapacitive devices, in which capacitance depends on the history of electrical excitation, offer a promising pathway toward non-volatile and analog memory elements for emerging electronic and neuromorphic systems. In this work, we present a comprehensive experimental demonstration of memcapacitive switching in inverse spinel NiFe₂O₄ thin films integrated into a symmetric Cu/NiFe₂O₄/Cu metal-oxide-metal device architecture. The NiFe₂O₄ layer, synthesized via a low-temperature hydrothermal route, exhibits a defect-rich inverse spinel structure with an optical band gap of ∼2.1 eV, favourable for polarization and charge trapping. The fabricated devices display clear memcapacitive signatures, including non-zero-crossing I-V hysteresis over a ± 5 V operating window, unpinched charge-voltage (Q-V) loops, and pinched capacitance-voltage (C V) characteristics, distinguishing the observed behaviour from filamentary memristive and ferroelectric switching. Electrical transport analysis reveals trap-controlled space-charge-limited conduction in the high-resistance state and polarization-enhanced transport in the low-resistance state, with current modulation spanning several orders of magnitude. Impedance spectroscopy performed over 5 Hz - 20 MHz confirms non-Debye dielectric relaxation and pronounced Maxwell-Wagner-type interfacial polarization. Time-resolved current data were further analyzed using MATLAB-based numerical reconstruction to extract the internal state variable and relaxation dynamics, yielding long relaxation times on the order of 10 −1 s, consistent with non-volatile capacitive memory. Cyclic voltammetry supports reversible redox-assisted nanoionic processes contributing to defect-mediated polarization. The combined materials characterization, device-level electrical analysis, and numerical modelling establish a coherent mechanism in which defect-assisted polarization, space-charge accumulation, and dielectric relaxation govern memcapacitive behaviour, positioning inverse spinel NiFe₂O₄ as a promising functional oxide for capacitive memory and neuromorphic applications.